In the use of optical radiation for various purposes, such as communications, it is desired to have a means to modulate the intensity of an optical beam. For example, in U.S. Pat. No. 3,748,597 issued to F. K. Reinhart (one of the inventors herein) on July 24, 1973, optical modulators were described using multilayers of different semiconductor materials of differing refractive indices. Moreover, in a paper published in Applied Physics Letters, Vol. 24, No. 6, pp. 270-272 (Mar. 15, 1974), authored by F. K. Reinhart, R. A. Logan (two of the inventors herein) and T. P. Lee, the transmission properties of epitaxial semiconductor rib waveguides were described. These rib waveguides were formed by geometrically selective anodization of epitaxial GaAs on Al.sub.x Ga.sub.1-x As layers, the "rib" being defined as that portion of the epitaxial layer of somewhat larger thickness than the remainder ("slab") of the epitaxial layer. Optical radiation propagating in the epitaxial layer along the rib direction tends to be confined in a portion of the epitaxial layer underlying the top of the rib surface (where the epitaxial layer has the larger thickness).
A semiconductor rib waveguide is thus useful in lateral confinement and waveguiding of a propagating optical beam. However, the height of the rib (equal to the difference in thickness between the rib portion and the remaining slab portion of the epitaxial layer) is an important parameter whose value must be carefully controlled in order to provide desired optical propagation in certain desired predetermined mode(s), ordinarily lowest order single mode propagation. Moreover, in order to have a means for modulating the optical radiation in accordance with an electrical signal, an electrode contact to the rib is required. In application, Ser. No. 557,250, filed on March 11, 1975, by R. A. Logan, J. L. Merz, F. K. Reinhart, and H. G. White, now U.S. Pat. No. 3,978,426, which issued on Aug. 31, 1976, a regrown epitaxial gallium aluminum arsenide layer was suggested for electrode contact to the semiconductor waveguide; however, such a contact as formed in an integrated optical device would require epitaxial regrowth on predetermined areas without affecting other semiconductive opitcal components, and would thus be difficult to accomplish in combination with rib waveguide structures. Moreover, the use of an electrical insulator layer, of relatively low refractive index such as silicon dioxide (n=1.5), for an optical buffer layer between an overlying metal electrode and the thin semiconductor waveguide suffers from the problem of undue electrical modulation field loss in the rib waveguide for the case of thick insulator buffer layers and the problem of pinholes in the oxide (which are filled by the ultimately overlying metal electrode) causing localized breakdown in the semiconductor waveguide for the case of thin insulator buffer layers. It would therefore be desirable to have a rib waveguide structure with a relatively easily controllable rib height and at the same time to have a simple and easily fabricated electrode, characterized by both low optical loss and good electrical conductivity, for modulating the optical properties of the rib waveguide.